Procedure and apparatus for the analysis of surface and/or depth profiles

ABSTRACT

The invention relates to a procedure for the analysis of surface and depthrofiles of partially or completely electrically non-conductive specimens using the direct bombardment method of the secondary neutral-particle mass spectrometry SNMS; to dissipate otherwise disturbing ion currents, it is being suggested that a high-frequency alternating voltage is applied to the underside of the specimen, the upperside of which is in contact with the low-pressure plasma used in the SNMS procedure, whereby the phase of this voltage is adjusted to such a time sequence that during the negative voltage part of the high-frequency period the specimen is bombarded by positive ions of a constant kinetic energy from the plasma and that the applied ion charge from the influx of plasma electrons is compensated during the positive part of the high-frequency period.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to mass spectrometry. More particularly thisinvention relates to secondary neutral-particle mass spectrometry.

2. Description of the Invention

In the known procedures concerned with the secondary neutral-particlemass spectrometry, SNMS, neutral atoms and molecules are released fromthe surface of a solid body by ion bombardment, are ionized by theelectron components of a low-pressure gas-discharge plasma and are thenanalyzed by a mass spectrometer. In an especially advantageous design ofSNMS, ions from the ionizing plasma are accelerated in accordance withthe direct bombardment technique toward the specimen by which method, inconjunction with an extremely planar adjustment of the equal potentialplanes in the ion acceleration path between plasma and specimen, alaterally highly homogeneous ion bombardment of the specimen surface canbe achieved. If by lowering the voltage between the specimen and areference electrode in the plasma, the ion energy is reduced to valuesaround 100 eV any of the atomic mixing processes in the specimen typicalfor higher energies can almost completely be avoided. When employed fordetermining deep profiles of concentrations, this method, in conjunctionwith a laterally highly homogeneous ion removal, yields an extremelyhigh depth resolution in the order of less than 1 nm.

These methods characteristic of the direct bombardment method of SNMScould, so far, be employed only on specimens whose electric conductivitywas sufficiently high such that a dissipation of the imposed ionbombardment current could occur. It is the objective of the presentinvention to enable using this method also on completely or partiallyelectrically non-conducive specimens.

SUMMARY OF THE INVENTION

The invention is based on the following principle:

When a negative voltage is applied to the underside of an isolatingspecimen or of a specimen that is mounted on an isolating surface (see(1) in FIG. 1a), this same voltage will also appear at the topside ofthe specimen through electrostatic induction. If the specimen is locatedwithin a plasma, the constant ion saturation current flowing onto thespecimen will compensate this surface voltage until the floating voltageis reached (FIG. 1b). During this `discharge time`, Δt₁, the specimen isbombarded by ions with ever decreasing energy. The determining factorsof Δt₁ are the specimen's capacity and the ion saturation current.

When a positive voltage is applied to the specimen (see (2) in FIG. 1a)the process described above repeats itself, now, however, with electronsflowing onto the specimen instead of ions. The corresponding `dischargetime` Δt₂ is shorter than Δt₁ due to the comparatively much higherelectron current (FIG. 1b).

If, instead, a rectangular-wave alternating voltage is applied with afrequency where the discharge time is large compared to the duration ofthe negative half period, the specimen surface cannot discharge itselfduring the negative half period (FIG. 2). During this period thespecimen is bombarded by highly monoenergetic ions and, thereby, eroded(sputtering). The length ratio depends on the requirements of thecorresponding analyses. It lies between 1 to 10² and 1 to 10⁵ and istypically (normal case) 1 to 10³.

By choosing a proper ratio of the positive voltage phase duration to thenegative voltage phase duration, ΔT(-):ΔT(+) in the high-frequencyperiod T (FIG. 2), the portion of T during which the specimen isbombarded by ions and which, therefore, is equal to the duration ofanalysis or the counting rate for the mass-spectrometric signals, can beoptimized. The requirement always remains that Δ₁ has to remain large incomparison to ΔT(-). ΔT(+) may be shortened to as low as Δt₂.

With the method described, the `direct bombardment mode (DBM)` of SNMS,formerly applicable only to electrically conductive specimens, can nowalso be applied to the analysis of dielectric specimens (isolators). Bychoosing the proper distance between the focusing orifice and thespecimen, a laterally homogeneous erosion of the specimen can beachieved since, during the negative half period in tho ion bombardmentphase, ion-optical conditions are achieved that are similar to the DBMof electrically conductive specimens. At small amplitudes of therectangular-wave high-frequency alternating voltage, i.e. at lownegative specimen voltages and, correspondingly, low ion bombardmentenergies, the same conditions are now achieved for dielectric specimenswhere the DBM on electrically conductive specimens leads to an extremelyhigh depth resolution. An example of this is shown in FIG. 3.

In one aspect of the invention the high-frequency alternating voltagehas a rectangular wave shape such that, during the ion bombardmentphase, a constant bombardment voltage is applied to the specimen (1)thus enabling ion-optical adjustments to achieve a laterally homogeneouserosion of the specimen (1) by ion bombardment.

In another aspect of the invention the positive portion ΔT(+), of thehigh-frequency voltage is about equal to the electron discharge time,Δt₂.

In yet another aspect of the invention the amplitude of thehigh-frequency voltage can be reduced down to below 100 V such that ahigh depth resolution can be achieved.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described on the basis of FIGS. 1 through 4:

FIG. 1a illustrates a time sequence of the rectangular-wave alternatingvoltage on the underside of the specimen.

FIG. 1b: shows a time sequence of the voltage on the specimen surface.The duration of a single half-period is larger than the `dischargetimes`, Δt₁ and Δt₂.

FIG. 2 shows a time sequence off the voltage on the specimen surface inthe case that the negative portion ΔT(-), of the high-frequency voltageis very small compared to the ion discharge time, Δt₁ (FIG. 1), and thatΔT(+) is comparable to the electron discharge time, Δt₂.

FIG.3 illustrates a concentration depth profile of a TaSi multi-layeredsystem (double layer thickness d=20 nm) determined by the SNMShigh-frequency method. In this measurement the multi-layeredsystem--itself being a conductor--was mounted on an electricallyisolating specimen holder. The frequency of the rectangular voltageemployed was 100 kHz. During the negative half period the specimen wasbombarded by 190 eV Ar⁺ ions in a direction perpendicular to thespecimen surface.

FIG. 4 is a schematic drawing of an SNMS device for performing analysesby the high-frequency method (see also DE-A-29 50 330). Located on theside of the plasma 2 and opposite to the specimen 1 are anelectron-optical arrangement 4, a quadrupole mass-spectrometer 5, a beamdiversion unit 6 as well as a secondary electron-multiplier tube 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The apparatus required for the SNMS analysis with the high-frequencymethod is identical to the SNMS/DBM design except for the onemodification of requiring a lead-in for the bombardment voltage which isnecessary for applying the high-frequency voltage (FIG. 4). The specimen1 to be analyzed is in direct contact with the plasma 2. In accordancewith this invention, a rectangular wave high-frequency voltage isapplied to the underside of the specimen 1, the voltage being created byan adjustable frequency rectangular-wave voltage generator 3 whosevoltage amplitude can be adjusted between 0 and several kV. By chosingthe proper frequency and amplitude a defined-bombardment energy can beachieved. The frequency must be so adjusted that no discharge worthmentioning occurs during the negative period (ΔT(-), see above). Withthis prerequisite fulfilled, a monoenergetic bombardment of the specimenis assured. The voltage on the surface of the specimen is determinedwith a measurement probe (not shown in FIG. 4) in order to enable a fineadjustment, if so required. This can be important in the depth profileanalysis and in the analysis of thin-layered systems where, in thecourse of sputtering (i.e with growing depth), a change of capacityoccurs. The measurement probe could be designed as a capacitative orinductive pickup or as a fine-point contact probo.

So far there is no known apparatus that combines high-frequencysputtering with the secondary neutral particle mass-spectrometry. Whenapplying high-frequency sputtering to isolators in plasmas one generallyuses sinusoidal alternating voltages in the MHz range. The essentialaspect of the present patent application is the use of arectangular-wave alternating voltage. This is prerequisite for achievinga monoenergetic bombardment energy together with a homogeneous specimenerosion in the case dielectric specimens.

Even if a high-frequency sinusoidal voltage in the order of 10 MHz isused which generally leads to a homogenisation of the bombardmentenergy, this still would not achieve the necessary homogenisation of thebombardment current required for a high depth resolution since aperiodically changing surface voltage also leads to a periodicallychanging depth of the space charge over the specimen. However, toachieve homogeneous specimen erosion a well defined time-constantrelationship between the voltage difference between specimen surface andplasma, the depth of the space charge layer.

We claim:
 1. Procedure for analyzing the surface and/or depth profile ofspecimens, said specimens being completely or partially electrically nonconductive, employing the direct bombardment method of the secondaryneutral-particle mass spectrometry SNMS, comprising the stepsof:applying a high-frequency alternating voltage to an underside of aspecimen, said specimen having an upperside in direct contact with alow-pressure plasma; wherein said alternating voltage has high frequencyperiods wherein, during negative voltage phases of said high-frequencyperiods, said upperside of said specimen is bombarded by positive ionsfrom said plasma with a constant kinetic energy for producing neutralatoms and molecules and, during positive voltage phases of saidhigh-frequency periods, said upperside of said specimen is bombarded byelectrons from said plasma for compensating the charge produced on saidspecimen by said positive ions.
 2. Procedure in accordance with claim 1further comprising the steps of:ionizing said neutral atoms andmolecules in said plasma; and analyzing said ionized atoms and moleculesin a mass spectrometer.
 3. Procedure in accordance with claim 1 or 2wherein the high-frequency alternating voltage has a rectangular waveshape such that, during the ion bombardment phase, a constantbombardment voltage is applied to the specimen thus enabling ion-opticaladjustments to achieve a laterally homogeneous erosion of the specimenby ion bombardment.
 4. Procedure in accordance with claim 3 wherein thenegative portion ΔT(-), of the high-frequency voltage is very smallcompared to the ion discharge time.
 5. Procedure in accordance withclaim 3 wherein the positive portion ΔT(+), of the high-frequencyvoltage is about equal to the electron discharge time, Δt₂.
 6. Procedurein accordance with claim 1 wherein an amplitude of the high-frequencyvoltage is less than 100 V whereby a high depth resolution is achieved.7. Procedure in accordance with claim 1, wherein a ratio between thenegative and the positive voltage phase of the high-frequency period isselected, whereby a laterally homogeneous ion bombardment is achievedover as long a part of the high-frequency period as possible. 8.Procedure in accordance with claim 1 wherein the underside of thespecimen, is connected to a generator which creates a rectangular-wavevoltage.